1. Field of the Invention
This invention relates to a fuel injection control system for internal combustion engines, and more particularly to a fuel injection control system of this kind, which controls a fuel injection amount so as to compensate for an amount of fuel adhering to the intake system of the engine.
2. Prior Art
An internal combustion engine of the type that fuel is injected into the intake pipe of the engine has the disadvantage that part of the injected fuel adheres to a wall surface of the intake pipe and hence a desired amount of fuel is not supplied into the combustion chamber of the engine. As one of solutions to overcome the disadvantage, a fuel injection amount control method is conventionally known, for example, from Japanese Patent Publication (Kokoku) No. 3-59255, which calculates a ratio of a fuel amount adhering to the wall surface of the intake pipe (adherent fuel amount ratio) and a ratio of a fuel amount carried off the wall surface of the intake pipe (carried-off fuel amount ratio), according to operating conditions of the engine, and corrects the fuel injection amount by adding an adherent fuel amount calculated from the adherent fuel amount ratio and subtracting a carried-off fuel amount calculated from the carried-off fuel amount ratio to and from the fuel injection amount, respectively, to thereby determine an amount of fuel to be supplied.
In the known fuel injection amount control method, the adherent fuel amount, which is used to calculate the carried-off fuel amount, is calculated based on a fuel amount to be supplied in the present fuel injection. However, in the case of split injection where fuel is injected a plurality of number of times in one cycle of the engine, the adherent fuel amount is calculated only for a fuel amount to be supplied in the first fuel injection but not taken into consideration for fuel amounts supplied in the second injection et seq. As a result, according to the above fuel injection amount control method, when the split injection is carried out, the accuracy of calculation of the adherent fuel amount is degraded.
To carry out adherent fuel amount-based correction even for the split injection, a fuel injection control system has been proposed, for example, by Japanese Provisional Patent Publication (Kokai) No. 3-23339, which calculates an adherent fuel amount for a fuel injection amount to be supplied in each injection of the split injection.
The above proposed fuel injection control system calculates an adherent fuel amount for each fuel injection, and calculates an amount of fuel to be supplied in each fuel injection, based on the thus calculated adherent fuel amount. Therefore, even when the split injection is carried out, the amount of fuel to be supplied in each fuel injection can be corrected by the adherent fuel amount, whereby a desired amount of fuel can be supplied into the combustion chamber of the engine.
However, the above proposed fuel control system calculates the adherent fuel amount for each fuel injection when the split injection is carried out. Therefore, this requires complicated arithmetic processing, which imposes a large burden on the software of the fuel control system.
More specifically, the above proposed fuel injection control system carries out the split injection when the fuel injection amount in the present cycle is larger than a predetermined value, such as during warming-up of the engine and during acceleration of the engine, and controls the fuel injection amount by calculating the adherent fuel amount for each fuel injection during the split injection. That is, in the proposed fuel injection control system, an additional injection is carried out in addition to a main injection to increase the fuel injection amount in the present cycle, during the split injection such as during acceleration of the engine. On this occasion, a calculation of an adherent fuel amount based on a fuel injection period for the main injection and a calculation of an adherent fuel amount based on a fuel injection period for the additional injection are carried out. This requires an increased amount of arithmetic processing as well as complicated arithmetic processing.